The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with open MRI systems with a C-shaped flux return path and will be described with particular reference thereto. It will be appreciated, however, that the present invention is useful in conjunction with other open systems, such as systems with H-shaped flux return paths, four poster arrangements, no ferrous flux return path, and the like, and is not limited to the aforementioned application.
In magnetic resonance imaging, a uniform main magnetic field typically denoted B0 is created through an examination region in which a subject to be examined is disposed. The resonance frequency in the field is determined by the field strength and the gyromagnetic ratio of dipoles to be resonated. With open magnetic resonance systems, the main magnetic field is typically vertical, perpendicular to the subject between upper and lower poles. A series of radio frequency (RF) pulses at frequencies coordinated to the resonance frequency are applied to two RF coils, one adjacent each pole, to excite and manipulate magnetic resonance. Gradient magnetic fields are conventionally produced by gradient coils to alter the resonance frequency in a preselected relationship with spatial position. The gradient coils are typically mounted between the RF coils and the poles. The magnetic resonance signals are detected with the two RF coils or localized coils and processed to generate two or three dimensional image representations of a portion of the subject in the examination region.
After pulses are applied by the RF coils, the excited dipoles decay back to the state of lowest energy. This is done at a characteristic frequency called the Larmor frequency. The Larmor frequency is a function of the total field strength, i.e., the sum of the main magnetic field and the superimposed gradient field. Thus, when the field strength varies, so does the Larmor frequency. When the field strength varies only with the applied gradients, the accurate calibration of field strength to spatial position within the examination region results in accurate images. However, when the field strength varies due to other, uncalibrated causes, ghosting and other artifacts in the final images result. When the main B0 field strength oscillates, the position of anatomical structures oscillate in the resultant image causing ghosting and other artifacts.
In open magnet systems, the poles are a set distance apart. This distance, along with the current strength in the superconducting magnet and other factors determine the strength of the main magnetic field in the examination region. If this distance changes, the B0 field strength changes causing problems with imaging as discussed previously.
In an open system having a 0.5 m opening, and a 0.23 T main field strength, a change of 1 ppm (part per million) in the aperture causes a 1 ppm change in the field strength, subsequently changing the Larmor frequency. Thus, a 0.5 xcexcm variation in the aperture varies the Larmor frequency by approximately 10 Hz. This is enough of a variance to cause ghosting in the final images. In higher field magnets, the frequency shift is significantly worse.
The aperture of an open system may change from any number of reasons. In a typical C-magnet system as described previously, an acceleration of only one thousandth of earth gravity (1 mg) produces a 1 ppm change in the aperture. Reasons such as people walking in the examination room or adjacent rooms, slamming doors, trucks in the street, and seismic activity can cause variations of this order of magnitude and higher. The acoustic reverberations of gradient and RF activity also cause vibration in the distance between the poles.
Previously, dampeners, such as rubber pads under the pole and flux return path, have been used to dampen environmental vibration forces. Such dampeners were effective for eliminating higher frequency components of vibrations, but lower frequency vibrations in the range of 5-20 Hz were less attenuated. A further disadvantage of using soft material to isolate vibrations is that the magnet is not supported in a firm position and it may shift from the original intended position.
Another method used is active vibration cancellation. These systems are massive and expensive. Typically large mechanical drivers are mounted under the pole and flux return path assembly. Environmental vibration is sensed and converted into counteracting physical movement. I this way, the actuators strive to create equal and opposite canceling vibrations. In spite of the expense, the ability of these systems to cancel vibrational movement is limited.
The present invention provides a new and improved method and apparatus that overcomes the above referenced problems and others.
In accordance with one aspect of the present invention, a magnetic resonance apparatus is given an imaging region is defined between upper and lower poles through which a main magnetic field is generated. A gradient coil assembly superimposes magnetic field gradients on the main magnetic field. A radio frequency coil assembly excites magnetic resonance in selected dipoles of a subject disposed in the imaging region. A reconstruction processor reconstructs received magnetic resonance signals into image representations. A force transducer is placed under the lower pole assembly to measure vibrations in the magnetic resonance apparatus.
According to a more limited aspect of the present invention, a vibration analyzer amplifies and processes signals to affect an operating frequency of a main oscillator to counteract the vibrations.
According to another aspect of the present invention, a method of magnetic resonance imaging is provided. A main magnetic field is induced through an examination region between a pair of pole assemblies. A subject is in the examination region. Magnetic resonance is excited, spatially encoded and received from selected dipoles within the subject. The signals are processed into a human readable form. Vibrations that alter the distance between pole assemblies are measured.
According to another aspect of the present invention, a magnetic resonance apparatus is provided. A substantially constant magnetic field is generated by a magnet between two pole assemblies, the field fluctuating due to an inconsistent distance between pole assemblies. A radio frequency transmitter induces resonance in selected dipoles that resonate at a resonance frequency, the frequency fluctuating as the main field fluctuates. A radio frequency receiver receives and demodulates the emitted resonance signals. A vibration sensor is connected with at least one of the pole assemblies. A vibration analyzing processor analyzes the sensed vibrations and determines compensations.
One advantage of the present invention is that it reduces imaging artifacts.
Another advantage of the present invention is that it provides images with sharp contrast.
Another advantage of the present invention is that it provides a more uniform and stable main magnetic field.
Another advantage is that it offers improved stability to an MR system.
Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.